1. Field of the Invention
The present invention relates to an optical switch device to be used as a primary component in an optical transport system, and more particularly to an optical switch device possible to totally reflect an incident light on an optical waveguide therein in accordance with a change in refractive index occurring owing to current application and a manufacturing method of the same.
2. Description of the Prior Art
Optical switch devices generally are used as principal components of an optical transport system possible to highly increase a transporting capacity of data and an operating speed therein, passing the limit of existing electronic switching systems.
Total reflection type of semiconductor optical switch has an operation characteristic, that, when current is applied to the optical switch, a refractive index of an optical waveguide layer therein is varied, or reduced. In detail, when current is applied to a part of an optical waveguide layer in such an optical switch, a difference of refractive index occurs between the current applied part and another part where current is not applied within the optical waveguide layer, and then a light propagating through the optical waveguide layer is totally reflected on an interface between the two parts in accordance with Snell's law. As a result, the propagating light is switched at the interface in the waveguide layer, and the switched light is propagated along another optical path. In order to obtain a total reflection of light in an optical waveguide, a change in refractive index must satisfy condition of following expression: EQU .DELTA.n.gtoreq.n(1-cos.theta.)
where n is refractive index of the waveguide layer, an is variation of refractive index of the waveguide layer caused due to a current application, and .theta. is a reflection angle of an incident light.
In order to manufacture an optical switch device provided with a totally reflecting interface therein, Conventional techniques most widely used presently can be summarized as three methods.
Firstly, one of the techniques is that, after performing crystallization on a semiconductor substrate to form a crystal layer, zinc impurity is partially diffused only into a light reflection surface of the crystal layer, as shown in FIG. 1A. This optical switch device provided with a partially diffused reflection surface is disclosed in "An 8 mm Length Nonblocking 4.times.4 Optical Switch Array", Areas in Commun., Vol. 6, pp. 1262-1266, 1988.
As shown in FIG. 1A, a first optical waveguide layer 2 is formed on a main surface of a semiconductor substrate 1. On the first optical waveguide layer 2, a clad layer 3 and a second optical waveguide layer 5 are sequentially formed. After zinc impurity is partially diffused into the second optical waveguide layer and the clad layer to form an impurity diffused portion 8, a well-known etching process in the art is performed so as to remove portions of the second optical waveguide layer, the clad layer and the first optical waveguide layer. Then, the first optical waveguide layer is removed to a predetermined depth thereof. Subsequently, a p-type front electrode and a rear electrode are respectively formed on the impurity diffused portion and a rear surface which is opposite to the main surface of the substrate 1. In construction of the optical Switch device manufactured thus, a light reflection surface is formed in the impurity diffused portion 8, as shown in FIG. 1A.
In such an optical switch device that is provided with a reflection surface, it is required that zinc impurity must be diffused into an optical waveguide layer, not exceeding the width of the waveguide. To reduce the width of the waveguide an area of ohmic contact is considerably limited within the optical switch device. Also, since zinc is diffused along a horizontal surface, width of a mask for zinc-diffusing must be considered, or reduced. If zinc has been diffused over a width of the waveguide, a current signal as carrier is dispersed over the waveguide. For this reason, the above-described optical switch device has the drawback that a current signal flowing in an optical waveguide can not be controlled effectively.
In addition, there is a making method of a slit type of optical switch device in which two diffusion steps are performed before crystallization on a semiconductor substrate and after crystallization so as to form an impurity diffused portion therein, as shown in FIG. 1B. The making method of this slit type optical switch device is well disclosed in "Appl. Phys. Lett.", Vol. 50, pp. 141-143, 1987). This slit type of optical switch device is provided to effectively control restraint of a current signal flowing through a waveguide therein.
The same components as those in FIG. 1A are indicated by the same reference numerals.
With reference to FIG. 1B, before formation of crystal on a main surface of a semiconductor, zinc is diffused into the substrate 1 using a mask so as to a first diffused portion 8A. Similarly to create crystallization as shown in FIG. 1A, a first optical waveguide layer 2, a clad layer 3 and a second optical waveguide 5 are sequentially formed on the substrate 1. Subsequently, zinc is diffused into the laminated layers 5 and 3 so as to form a second diffused portion 8B, and a front electrode 6 and a rear electrode 7 are formed on the second diffused portion 8B and a rear surface opposite to the main surface of the substrate 1, respectively. As a result, the slit type optical switch device has a p/n/p/n current blocking layer, and hence restraint of a current signal flowing therein can be effectively controlled.
However, such a slit type of optical switch device has the drawback that a lithographic alignment technique having precision of 1 .mu.m or less is required to fabricate such an optical switch device, and two diffusing steps must be performed under several complicated conditions such as accurate control in quantity of a diffusing material or a precise temperature. Similarly, the slit type of the optical switch device has another drawback that reduction of the width of the waveguide and an area of ohmic contact is significantly limited.
Finally, there is an InGaAsP/InP optical switch device having a semi-insulating InP current blocking layer, as shown in FIG. 1C. This optical switch device is disclosed in "InGaAsP/InP Optical Switches Embedded with Semi-Insulating InP Current Blocking Layers", Sel. Areas in Commun., Vol. 16, pp. 1199-1204, 1988.
The same components as those in FIG. 1B are indicated by the same reference numerals.
As shown in FIG. 1C, on a main surface of a semiconductor substrate 1 a first optical waveguide layer 2 and a current blocking layer 4 are sequentially formed. By an etching process, a portion of the current blocking layer 4 is removed to form an open portion. Subsequently, re-crystallization steps are performed so as to form a clad layer 3' and a second optical waveguide layer 5' in the open portion. A front electrode 6 and a rear electrode 7 are respectively formed on the second optical waveguide layer 5' and a rear surface opposite to the main surface of the substrate 1, respectively.
Because such an InGaAs/InP optical switch device has an InP semi-insulating layer produced by performing crystallization twice, a current flowing therein can be effectively blocked.
However, this optical switch device has the same drawbacks as that of the above-mentioned switches. Also, in the InGaAs/InP optical switch device, since zinc is diffused into the waveguide to form an impurity diffused portion and a front electrode 6 is formed only on the diffused portion, a contact area between the diffused portion and the front electrode 6 is considerably limited and hence ohmic characteristic is lowered. As a result, a high current signal of 90 mA or more is required to execute a switching operation in this InGaAs/InP optical switch device, and thereby current consumption is further increased in quantity.